1. Field of the Invention
The present invention relates to a recording head for such type of ink jet recording apparatus in which a recording liquid as generally referred to as ink is jetted from an orifice, in a form of droplets, toward the surface of a recording material such as paper to effect printing on it. More particularly, the present invention is directed to improvements in the orifice portion of such ink jet recording head.
2. Description of the Prior Art
At present, various recording methods are known and used. Among others, ink jet recording method has particular advantages. This is a non-impact recording method according to which almost no noise is generated during recording operation. It enables high speed recording. In addition, recording can be accomplished on any common paper without need of particular fixing treatment. In view of these advantages many ink jet recording systems have been proposed and many attempts have been made to further improve them. Some of them have already been accepted in the market and some of them are still under development to put them to practical use.
In brief, ink jet recording method is a recording method wherein recording liquid, namely ink is jetted in a form of ink droplets flying toward a recording member such as paper on which the ink droplets are deposited to effect printing. Known ink jet recording processes are grouped into several types in accordance with the method used to generate recording ink droplets, the method used to control the flying course of the droplets and the like.
One typical type is generally called continuous type which is disclosed, for example, in U.S. Pat. Nos. 3,596,275 (Sweet Process) and 3,298,030 (Lewis and Brown Process). According to the continuous type of ink jet recording process, a stream of ink droplets charged with electric charge is generated while controlling the charge by a continuous oscillation generating method. The ink droplets with a controlled amount of electric charge are jetted toward a recording member. To control flying course of the ink droplets, there are disposed a pair of deflecting electrodes to which a uniform electric field is being applied. The ink droplets fly toward the recording member passing through between the deflecting electrodes.
Another typical type opposable to the above is that generally called on-demand type which is disclosed, for example, in U.S. Pat. No. 3,747,120 (Stemme Process). According to the ink jet recording method this type there is used a recording head having an orifice for jetting ink from it and piezo-oscillator mounted on the head. An electric recording signal is applied to the piezo-oscillator by which the electric signal is transformed into a mechanical oscillation of the piezo-oscillator. Every time when demand is made, ink droplets are jetted from the orifice toward a recording member in accordance with the mechanical oscillation of the piezo-oscillator.
An example of recording head commonly used in the known ink jet recording systems outlined in the above is shown in FIG. 1.
The recording head generally designated by 1 comprises a conduit pipe 2 made of suitable material such as glass, ceramics or metal and a piezo element 3 disposed around the the pipe 2. The conduit pipe 2 forms a part of the flow passage 6 for ink IK and has a fine hollow. The piezo element 3 constitutes means for jetting the ink filled in the pipe 2 from a jet orifice 4. The piezo element 2 should be considered as an example of such ink jetting means.
At one end of the conduit pipe 2 opposite to the orifice 4, the conduit pipe 2 is connected with a tube 5 made of, for example, polyvinyl chloride. The tube 5 forms another part of the flow passage for ink IK and extends to an ink tank not shown. Ink IK is supplied to the conduit pipe 2 from the ink tank through the tube 5 in the direction of arrow P.
In the recording head 1 having the above described structure, physical properties of the surface around the jet orifice 4 are of critical importance for obtaining a stable jet of ink IK from the orifice 4 constantly.
In the shown example, the orifice 4 is formed by one end portion of the conduit pipe 2 integrally with the pipe. However, the orifice may be formed by a separate orifice plate having an opening of predetermined diameter fixed to the end portion of the pipe 2. In either case, there arise difficult problems in jetting ink from the orifice. In use of the recording head 1 the ink IK sometimes flows into the area of the outer surface around the orifice 4 and forms a pool of ink there. If once formed, such a pool of ink disturbs the stability of jet of ink from the orifice. The flying course of ink droplets jetted from the orifice is made deviated from the determined regular direction by it. Furthermore, the flying direction of ink droplets varies every time of ink jet. It is no longer possible to obtain a stable jet of droplets. Therefore, no good recording can be assured. If the whole surface area surrounding the orifice 4 is covered with a film of ink IK, then there occurs so-called splash phenomenon by which the ink is scattered, which also prevents stable recording. In the worst case, the jet of ink droplets from the orifice gets blocked with the growth of such pool of ink around the orifice.
To solve the above problem it has been already proposed to treat the outer surface of the recording head surrounding the jet orifice with silicone oil or the like to render the surface water repellent. For example, reference is made to Japanese Utility Model Application Publication No. 36,188/1973. However, known agent for above treatment is poor in adhesive property to glass, metal or other material by which the orifice is formed. Therefore, it lacks durability and the desired effect of the treatment is obtainable only for a short time after the treatment. Moreover, the treatment agent possesses not only fluidity but also solubility to the solvent commonly used in the recording ink composition. Due to these properties, the treating agent is washed away by the recording ink and therefore its effect can not last long. The treating agent frequently mixes in the recording ink and results in change in composition of the ink which may have adverse effects on the performance of the ink jet recording.
Another disadvantage of the known treating agents are found in their unsatisfactory liquid repellency. For example, silicone system treating agent has an adequate repellency to aqueous system inks. But, it exhibits no repellency to organic solvent system inks such those of alcohol system, ketone system and ester system.